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December 22, 2014

Homework Help: Phase diagram

Posted by Sheryl on Sunday, November 12, 2006 at 8:55pm.

Question: A solid substance has a vapor pressure of 800 mm Hg at its melting point (80 degrees C). Describe how the solid behaves as temperature is raised from room temperature to 80 C while the atmospheric pressure is held constant at 760 mm Hg.

I think that the the substance will begin to melt a the temp goes up and by the time it hits 80, it will be in the liquid phase because the vapor pressure is below 800 mm Hg.

Is this correct?

Thanks from Sheryl

Vaporization is when a substane turns from a liquid to a gas. Thus, I believe that since the unknown substance's vaporization point is 80 degrees Celsius it will turn into a gas at this point because the temperature is being raised, forcing the intermolecular particles to spread even further apart and to have more entropy (randomness), then a liquid. The particles will also move more rapidly.

Since the vapor pressure of 800 mm Hg is never attained - it stays at 760 - wouldn't it not become a gas?

Sheryl

You are right, but if the vapor pressure was higher it would be a gas. =)

Does the solid go from solid to liquid to gas?

Sheryl

We don't know because we haven't been given the complete phase diagram. It could, or it could go directly to a gas.

Sheryl: The vapor pressure of the solid is dependent on temperature. Somewhere just before it melts at 80 deg, the vapor pressure of the solid is equal to atmospheric pressure, and it vaporizes in an instant (like a solid boiling) to a gas. It never melts, as it does not get to the mp.

Thank you for that clarification. I couldn't get that it doesn't go liquid before vaporizing, now I do.

Sheryl

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